The extraction of copper, cadmium and related metals (M2+) with poly(sodium acrylate - acrylic acid) PAA hydrogels has been studied. pH variations are consistent with a cation exchange process. Saturation of the gel is achieved for a metal/carboxylate ratio R ≈ 1/2 and a gel swelling of ≈ 40 which is that of the uncharged gel : (-COO)2M complexes are expected to be formed, but also complexes of higher stoichiometry (R = 1/3 for Cu and Cd, R = 1/4 for Pb) at low metal concentrations. The selectivity is that observed in liquid-liquid extraction of metal cations with fatty carboxylic acids (Pb > Cu > Cd > Zn > Ni ≈ Co). Metal stripping from the gel is readily achieved by washing with 0.1 M HNO3. PAA hydrogel extraction allows to remove cadmium from a diluted aqueous solution down to a final concentration of 5 ppb.
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Rifi et al. (1994) studied this question.
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